Page:EB1911 - Volume 23.djvu/166

ANATOMY] occipital, part of the opisthotic, the quadrate, and part of the pterygoids, form the bony borders of the middle ear—cavity, which is open behind; through it extends, horizontally the columellar rod, received with its outer portion by a notch on the posterior side of the quadrate. This is of very complicated shape. Its outer margins form most of the tympanic frame; the posterior margins being curved backwards leave a wide notch behind in the Cryptodira and in Sphargis, but in the Pleurodira this part of the quadrate is transformed into a trumpet, the rim of which, forming a complete ring, carries the tympanic membrane. The tympanic cavity thus formed often leads into a deep recess which extends into the hollowed-out squamosal (e.g. in Testudo) towards the opisthotic and bears some resemblance to the intricate tympanic recesses which pervade that region of the crocodile's skull. With its upper anterior and inner portion the quadrate joins the large proötic bone which is usually completely fused with the rest of the opisthotic, but in Sphargis it remains separate, and in this turtle the sutures between the otic bones and the supraoccipital also persist. In front of the proötics the bony lateral walls of the brain-case end in Sphargis, but in most of the other Chelonians bony alisphenoids are represented by a pair of epipterygoids which rest upon short upward processes of the pterygoids and are joined by much longer, rather thin, but broad descending lamellae from

the parietal. They represent of course the columellae cranii or pterygoidal columellae; if they are of alisphenoidal origin the term epipterygoids is a misnomer; the same applies to these structures in other reptiles. Through the space enclosed by the pterygoid, basioccipital, opisthotic and quadrate, enters the cranial carotid artery, sometimes piercing the posterior rim of the pterygoid; then the canal runs along the dorsal side of this bone and opens near the cranial columella. The arcades over the temporal region are most variable. Potentially Chelonians possess all the three arcades of the crocodiles, but it so happens that never more than one fenestra is present. The false roof over the temporal region is most complete in Sphargis and in the Chelonidae. Excepting Sphargis the supraoccipital extends far beyond the back of the cranium in shape of a long unpaired crest, which never diverges, or sends out lateral processes, but it is joined, and partly overlaid for a great part of its length, by the parietal in Chelonidae and Sphargis. In these genera the much-enlarged parietal, the equally large postfrontal, with the squamosal behind, the jugal below, and a large quadrato-jugal, form one continuous bony roof over the whole temporal fossa, which is widely open behind, the space being bordered by supraoccipital, opisthotic, squamosal and parietal. All other Chelonians show a great reduction of this roof. The parietal does not send out dorsolateral expansions; and the postfrontal likewise forms no expansions. It joins the rather short malar, forming the posteriororbital bridge, which posteriorly is connected by the quadrato-jugal with the upper portion of the quadrate and with the squamosal. The latter rests upon the quadrate and is in no connexion with the parietal. Consequently the whole temporal fossa is quite open. The horizontal bridge or arcade is to a certain extent homologous with the infra-temporal arcade. All the bones which border the temporal fossa vary much in extent. The greatest reduction has taken place in Cistudo and in Geoemyda, the latter an Indian genus of Testudinidae, in which the quadrato-jugal is lost, leaving a wide gap in the horizontal arcade.—

The Chelonians form an instructive parallel to mammalian conditions by the broad contact of the squamosal with the malar, e.g. in Chelone, whilst the quadrato-jugal, having in all Chelonians lost its original ventral connexion with the jugal, may actually get lost as in all the